COINRG                                                          I. Kunze
Internet-Draft                                                 K. Wehrle
Intended status: Informational                    RWTH Aachen University
Expires: September 10, 2020                               March 09, 2020

       Transport Protocol Issues of In-Network Computing Systems


   Today's transport protocols offer a variety of functionality based on
   the notion that the network is to be treated as an unreliable
   communication medium.  Some, like TCP, establish a reliable
   connection on top of the unreliable network while others, like UDP,
   simply transmit datagrams without a connection and without guarantees
   into the network.  These fundamental differences in functionality
   have a significant impact on how COIN approaches can be designed and
   implemented.  Furthermore, traditional transport protocols are not
   designed for the multi-party communication principles that underlie
   many COIN approaches.  This document discusses selected
   characteristics of transport protocols which have to be adapted to
   support COIN functionality.

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   This Internet-Draft will expire on September 10, 2020.

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   ( in effect on the date of
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Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   2
   2.  Addressing  . . . . . . . . . . . . . . . . . . . . . . . . .   3
   3.  Flow granularity  . . . . . . . . . . . . . . . . . . . . . .   3
   4.  Authentication  . . . . . . . . . . . . . . . . . . . . . . .   4
   5.  Security  . . . . . . . . . . . . . . . . . . . . . . . . . .   5
   6.  Advanced Transport Features . . . . . . . . . . . . . . . . .   5
     6.1.  Reliability . . . . . . . . . . . . . . . . . . . . . . .   5
     6.2.  Flow/Congestion Control . . . . . . . . . . . . . . . . .   7
   7.  Security Considerations . . . . . . . . . . . . . . . . . . .   9
   8.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .   9
   9.  Conclusion  . . . . . . . . . . . . . . . . . . . . . . . . .   9
   10. Informative References  . . . . . . . . . . . . . . . . . . .   9
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  10

1.  Introduction

   A fundamental design choice of the Internet is that the network
   should be kept as simple as possible while complexity in the form of
   processing should be located on end-hosts at the edges of the
   network.  This choice is reflected in the end-to-end principle which
   states that end-hosts directly address each other and perform all
   relevant computations while the network only delivers the packets
   without modifying them.  Transport protocols are consequently
   designed to facilitate the direct communication between end-hosts.

   In practice, the end-to-end principle is often violated by
   intransparent middleboxes which alter transmitted packets, e.g., by
   dropping or changing header fields.  Contrary to that, computing in
   the network (COIN) encourages explicit computations in the network
   which introduces an intertwined complexity as the computations on the
   end-hosts depend on the functionality deployed in the network.  It
   further challenges traditional end-to-end transport protocols as they
   are not designed to address in-network computation entities or to
   include more than two devices into a communication.  Some of these
   problems are already presented in [I-D.draft-kutscher-coinrg-dir-01].

   This draft intends to discuss potential problems for traditional
   transport protocols in more detail to raise questions that the COIN

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   community needs to solve before a widespread application of COIN
   functionality is sensible.  Collaboration with other IRTF and IETF
   groups, such as PANRG, the IETF transport area in general, or the
   LOOPS BOF, can help in finding suitable solutions.

2.  Addressing

   The traditional addressing concept of the Internet is that end-hosts
   directly address each other with all computational intelligence
   residing at the network edges.  With COIN, computations move into the
   network and need to be integrated into the established
   infrastructure.  In systems where the whole network is under the
   control of the network operator this integration can be implemented
   by explicitly adjusting the communication schemes based on the COIN
   functionality.  Considering larger scales, e.g., Internet-wide
   deployment, this approach of manually adjusting traffic patterns and
   applications to correctly incorporate changes made by the network is
   no longer feasible.

   What is needed are ways to specify which kind of functionality should
   be applied to the transmitted data on the path inside the network and
   maybe even where or by whom the execution should take place.  Such
   functionality could for example be implemented using an indirection
   mechanism which routes a packet along a pre-defined or dynamically
   chosen path which then realizes the desired functionality.  Related
   concepts are Segment and Source Routing as well as (Service/Network)
   Function Chaining/Composition.

   Main challenges/questions are:

   1.  How should end-hosts address the COIN functionality?

   2.  How can the end-hosts influence where or by whom the
       functionality is executed?

   3.  How can devices which do not implement COIN functionality be
       integrated into the systems without breaking the COIN or legacy

   Another question is how the transmitted data is to be treated by the
   devices implementing the functionality.

3.  Flow granularity

   Core networking hardware pipelines such as backbone switches are
   built to process incoming packets on a per-packet basis, keeping
   little to no state between them.  This is appropriate for the general
   task of forwarding packets, but might not be sufficient for COIN as

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   information that is needed for the computations can be spread across
   several packets.  In a TCP stream, for example, data is dynamically
   distributed across different segments which means that the data
   needed for application-level computations might also be split up.  In
   contrast to that the content of UDP datagrams is defined by the
   application itself which is why the datagrams could either be self-
   contained or information can be cleverly distributed onto different
   datagrams.  The common scheme is that different transport protocols
   induce different meanings to the packets that they send out which
   needs to be accounted for in COIN elements as they have to know how
   the received data is to be interpreted.  There are at least three
   options for this.

   1.  Every packet is treated individually.  This respects the
       possibilities that are already offered by all networking

   2.  Every packet is treated as part of a message.  In this setting,
       the packet alone does not have enough information for the
       computations.  Instead, it is important to know the content of
       the surrounding packets which together form the overall message.

   3.  Every packet is treated as part of a byte stream.  Here, all
       previous packets and potentially even all subsequent packets need
       to be taken into consideration for the computations as the
       current packet could, e.g., be the first of a group of packets, a
       packet in the middle or the final packet.

   The flow granularity consequently has a significant impact on how
   computations can be performed and where.  Apart from how the COIN
   elements should treat the transmitted data, another important aspect
   is how it can be ensured that the end-hosts know who has altered the
   data and how.

4.  Authentication

   The realisation of COIN legitimizes and actively promotes that data
   transmitted from one host to another can be altered on the way inside
   the network.  This opens the door for foul play as all intermediate
   network elements - no matter if they are malicious or misbehaving by
   accident, COIN elements, or 'traditional' middleboxes - could simply
   start altering parts of the original data and potentially cause harm
   to the end-hosts.  What is needed are mechanisms with which the
   receiving host can verify (a) how and (b) by whom the data has been
   altered on the way.  In fact, these might very well be two distinct
   mechanisms as one (a) only focusses on the changes that are made to
   the data while (b) requires a scheme with which COIN elements can be
   uniquely identified (could very well relate to Section 2) and

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   subsequently authenticated.  The Proof of Transit
   [I-D.draft-ietf-sfc-proof-of-transit-04] concept of the SFC WG could
   be applicable for proving that a packet has indeed passed the desired
   COIN elements, although it does not provide means to validate which
   changes were made by the known nodes.

   Main challenges/questions are:

   1.  How are changes to the data within the network communicated to
       the end-hosts?

   2.  How are the COIN elements that are responsible for the changes
       communicated to the end-hosts?

   3.  How are changes made by the COIN elements authenticated?

5.  Security

   Many early COIN concepts require an unencrypted transmission of data.
   At the same time, there is a general tendency towards more and more
   security features in communication protocols.  QUIC, e.g., encrypts
   all payload data and almost all header content already inside the
   transport layer.  This makes current COIN concepts infeasible in
   settings where QUIC connections are used as the COIN elements do not
   have access to any packet content.  Using COIN thus also depends on
   how well security mechanisms like encryption can be integrated into
   COIN frameworks.

   Together, the four aspects presented in Section 2 to Section 5 form a
   set of fundamental properties for a basic transport-compatible
   realization of COIN.  In the following, we briefly discuss selected
   additional transport features to create awareness for the
   multifaceted interaction between the transport protocols and COIN

6.  Advanced Transport Features

   Depending on application needs, different transport protocols provide
   different features.  These features shape the behavior of the
   protocol and have to be taken into account when developing COIN
   functionality.  In this section, we focus on the impact of
   reliability as well as flow and congestion control.

6.1.  Reliability

   Applications require a reliable transport whenever it is important
   that all data has been transmitted successfully.  TCP[RFC0793]
   provides such a reliable communication as it first sets up a

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   dedicated connection and then ensures the successful reception of all
   data.  In contrast, UDP[RFC0768] is a connectionless protocol without
   guarantees and COIN elements working on UDP transmissions must be
   robust to lost information.  This is not the case for applications on
   top of TCP, but the retransmissions and the TCP state, which TCP uses
   to achieve the reliability, make packet processing for COIN more
   complex due to at least three reasons.

   The concept of retransmissions bases on the end-to-end principle as
   retransmissions are performed by the sender if it has determined that
   the receiver did not receive the corresponding original message.
   Both participants can then act knowing that parts of the overall data
   are still missing.  For simple COIN elements, which are not aware of
   the involved TCP states and which do not track sequence numbers, it
   is difficult to identify (a) that a packet in the sequence is missing
   and (b) that a packet is a retransmission.  One question is whether
   COIN elements should incorporate an understanding for retransmissions
   on the basis of existing transport mechanisms or if a COIN-capable
   transport should include dedicated signals for the COIN elements.

   Apart from challenges in identifying retransmissions, there is also
   the fact that they are sent out of order with the original packet
   sequence.  Depending on the chosen flow granularity (see Section 3),
   COIN elements might have to hold contextual information for a
   prolonged time once they identify an impeding retransmission.
   Moreover, they might have to postpone or cancel computations if data
   is missing and instead schedule later computations.  The main
   question arising from this is: to what extent should COIN elements be
   capable of incorporating retransmissions into their computation
   schemes and how much additional storage capabilities are required for

   When incorporating COIN elements into the retransmission mechanisms,
   it is also an interesting question whether it should be possible to
   request or perform retransmissions from COIN elements.  Considering a
   setting with COIN elements that are capable of detecting missing
   packets and retransmission requests, it might improve the overall
   performance if the COIN element directly requests or performs the
   retransmission instead of forwarding the packet/request through the
   complete sequence of elements.  In both cases, the aforementioned
   storage capabilities are relevant so that the COIN elements can store
   enough information.  The general question, i.e., which nodes in the
   sequence should do the retransmission, has already been worked on in
   the context of multicast transport protocols.

   Depending on the extent of realization of the presented
   retransmission features, COIN elements might almost have to implement
   some of TCP's state to fulfil their tasks.  Considering that

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   different COIN elements have different computational and storage
   capacities, it is very likely that not every form of transport
   integration into COIN can be supported by every available COIN
   platform.  The choice of devices included into the communication will
   hence certainly affect the types of transport protocols that can be
   operated on the COIN networks.

   Challenges/questions regarding reliability are:

   1.   Should COIN elements be aware of retransmissions?

   2.   How can COIN elements identify missing packets or

   3.   Should COIN elements be explicitly notified about

   4.   To what extent should COIN elements be capable of incorporating
        retransmissions into their computation schemes?

   5.   How much storage capabilities are required for incorporating

   6.   How can COIN elements incorporate missing packets into their

   7.   How to deal with state changes in COIN elements caused by data
        lost later in the communication chain and then retransmitted?

   8.   Should COIN elements be capable of requesting retransmissions/
        answering retransmission requests?

   9.   Which devices should perform retransmissions?

   10.  Do COIN elements have to keep transport state?

   11.  How much transport state do COIN elements have to keep?

   Another aspect is flow and congestion control to avoid overloading
   the receiving end-host and the network.

6.2.  Flow/Congestion Control

   TCP incorporates mechanisms to avoid overloading the receiving host
   (flow control) and the network (congestion control) and determines
   its sending rate as the minimum value of what both mechanisms
   determine as feasible for the system.  This approach is based on the
   notion that computing and forwarding hosts are separated and is

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   challenged by the inclusion of COIN elements, i.e., computing nodes
   in the network.

   Flow control bases on explicit end-host information as the
   participating end-hosts notify each other about how much data they
   are capable of processing and consequently do not transmit more data
   as the other host can handle.  This only changes if one of the end-
   hosts updates its flow control information.

   Congestion control, on the other hand, interprets volatile feedback
   from the network to guess a sending rate that is possible given the
   current network conditions.  Most congestion control algorithms
   hereby follow a cyclical procedure where the sending end-hosts
   constantly increase their sending rate until they detect network
   congestion.  They then decrease their sending rate once and start to
   increase it again.

   In this traditional two-fold approach, loss, delay, or any other
   congestion signal (depending on the congestion control algorithm)
   induced by COIN elements (only in case that they are the bottleneck)
   is interpreted as network congestion and thus accounted for in the
   congestion control mechanism.  This means that the sending end-host
   may repeatedly overload the computational capabilities of the COIN
   elements when probing for the current network conditions instead of
   respecting general device capabilities as is done by flow control.

   Consequently, the question arises whether COIN elements should be
   able to participate in end-to-end flow control.

   Main challenges/questions are:

   1.  Should COIN elements be covered by congestion control?

   2.  Should COIN elements be able to participate in end-to-end flow

   3.  How could a resource constraint scheme similar to flow control be
       realized for COIN elements?

   All in all, there is a wide range of non-essential transport features
   which offer improved performance in certain settings and for certain
   application combinations.  However, as presented, it is likely that
   not all of the features/types of transport protocols can be supported
   on every COIN element.  It might make sense to define different
   classes of COIN-ready transport protocols which can then be deployed
   depending on the concretely available networking/hardware elements.
   Alternatively, each of the features could be treated separately and

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   they could then be composed based on the demands of an application

   Overall, the general question summarizing all of the other
   challenges/question is:

   Which transport features should be supported by COIN, how can they be
   identified and how can they be provided to application designers?

7.  Security Considerations


8.  IANA Considerations


9.  Conclusion

   The advent of COIN comes with many new use cases and promises
   improved solutions for various problems.  It is, however, not
   directly compatible with the end-to-end nature of transport
   protocols.  To enable a transport-based communication, it is thus
   important to answer key questions regarding COIN and transport
   protocols, some of which are raised in this document.

10.  Informative References

              Brockners, F., Bhandari, S., Mizrahi, T., Dara, S., and S.
              Youell, "Proof of Transit", draft-ietf-sfc-proof-of-
              transit-04 (work in progress), November 2019.

              Kutscher, D., Karkkainen, T., and J. Ott, "Directions for
              Computing in the Network", draft-kutscher-coinrg-dir-01
              (work in progress), November 2019.

   [RFC0768]  Postel, J., "User Datagram Protocol", STD 6, RFC 768,
              DOI 10.17487/RFC0768, August 1980,

   [RFC0793]  Postel, J., "Transmission Control Protocol", STD 7,
              RFC 793, DOI 10.17487/RFC0793, September 1981,

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Authors' Addresses

   Ike Kunze
   RWTH Aachen University
   Ahornstr. 55
   Aachen  D-50274

   Phone: +49-241-80-21422

   Klaus Wehrle
   RWTH Aachen University
   Ahornstr. 55
   Aachen  D-50274

   Phone: +49-241-80-21401

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